Independent write

[Joachim Worringen]

Rob Ross:
> > One might define an "atomic mode" for pNetCDF, like in MPI-IO.
>
> Ack no!!!!  I *hate* the MPI-IO atomic mode!!!  We'd have to augment the
> interface to have an append mode as well, because of the race condition
> mentioned.

I didn't say that I'm an "atomic fan" ;-). But there are reasonably efficient 
ways to implemented it under certain conditions. Will PVFS2 provide atomic 
mode? ;-)

> > MPI-2 one-sided communication could be a way to implement the required
> > synchronization (doulbe-locking two windows to access some data in one
> > of the windows atomically like fetch&increment - not nice, but works).
> > It's efficiency would depend on the progress characteristics of the
> > underlying MPI implementation and/or the interconnect and/or the
> > communication behaviour of the application, but it would work anyway.
>
> Yeah, this is a clever way to implement this sort of functionality.  
> There is at least one group looking at this sort of thing for consistent
> caching in the MPI-IO layer.

Yes, you can do a lot of things with this (see above).

[...]
> Maybe I should add a section to the document on "implementing append
> operations in PnetCDF", describing a couple of approaches and giving some
> code?  I would much rather spend a little time doing this and helping
> people do things the right way.

I share your concerns (people tend to do it the easy way!), and think this is 
the better way to handle this issue, too. It's necessary to change thinking 
if going parallel.

> > As many interconnect today allow RMA, the performance of passive
> > synchronization should become better with the MPI implementations
> > exploiting such capabilities.
>
> Agreed.  There are definitely lots of opportunities there!  RMA
> implementations need to be more wide-spread though...

Doesn't everything next to Ethernet support some sort of RMA today? And even 
for Ethernet, there are some approaches to do such staff (forgot the name of 
this). And we know that "Ethernet always wins"... The time might come which 
will make it advisable to update or extent the MPI one-sided specs.

 Joachim

-- 
Joachim Worringen - NEC C&C research lab St.Augustin
fon +49-2241-9252.20 - fax .99 - http://www.ccrl-nece.de